It's obviously difficult to predict whether a heel turn can lead to "the biggest heel of all time", but I definitely think that John Cena is capable of becoming a great heel. And if done properly (which is the KEY), I think fans will be very satisfied with the results. So let's try to come up with a specific, logical, and practical way to turn John Cena heel before we debate how successful a heel he can actually become. Remember, you have to learn to walk before you can run.
So after much thought, I believe I have a solid way to turn John Cena heel. And while it is along similar lines to many of the proposed ideas we have heard before, it varies just slightly.
Most people believe that the best way to turn John Cena into a MAJOR heel is to screw the Undertaker over at Wrestlemania and end his undefeated streak. The 3 main constants are always the Undertaker, Wrestlemania, and "the streak". The MAJOR downside; however, is that you're ruining something that has been building for nearly 20 years and has become the staple of Wrestlemania. If you decide to end "the streak", future Wrestlemanias (with or without the Undertaker) will take a significant hit unless Wrestlemania can regain the mystique it once had.
I am on board; however, with many of the factors talked about above...just not all of them. I think turning John Cena heel at Wrestlemania is absolutely the way to go. I also believe the Undertaker is the one, current superstar who can instantly turn someone heel...in fact, he can turn them into a MAJOR heel. I don't agree with taking the Undertaker's streak; however. Maybe it's because the Undertaker is my favorite superstar, maybe it's because he is a living legend, maybe because it's because the streak has taken on a life of it's own. But as much as I'd like to see a new phase of John Cena's career, I don't believe it should come at the expense of the Undertaker's streak.
So here is what I propose. Much like Shawn Michaels vs. The Undertaker, I think seeds need to be planted months prior to Wrestlemania. Whether it be in the ring or backstage, the two need to cross paths (and I don't care how they decide to bring the two together or what the storyline is because that will become irrelevant in the grand scheme of things). A slow build needs to happen, though. From there, John Cena will face the Undertaker at Wrestlemania. And it will be FACE vs. FACE. Yes, that's right! John Cena will not be a heel prior to Wrestlemania. The Undertaker will say that he respects John Cena, but he will not be able to end the streak. John Cena will say that he respects the Undertaker, too and considers him the legend of legends, but he will never back down, never give up, and will defeat the Undertaker. It will basically be a super face vs. super face match.
When the match actually takes place, it will be very competitive and long. Of course, we'll have numerous close calls and multiple finishing moves will be hit. In the end, though, the Undertaker will defeat John Cena (maybe, even by submission). The streak will remain intact. From there, the Undertaker will celebrate to close out Wrestlemania, as usual. As John Cena finally struggles to his feet, he will approach the Undertaker and offer a handshake. The Undertaker will accept it, and everything will be just peachy. John Cena will appear to leave the ring, but he'll change his mind at the last minute and brutally attack the Undertaker. He'll hit him with a chair, slap on the STFU, etc. It'll be a vicious assault (PG style, of course). Wrestlemania will close with everybody in shock.
What you will have essentially done is turned John Cena into a super heel, kept the Undertaker's streak intact, and given a legitimate reason for the Undertaker to take off significant time after Wrestlemania (as he usually likes to take months off after Wrestlemania). From there, John's heel character can grow. As stated several times, many people like the idea of John Cena's character being frustrated over the fan's lack of support for him. Why do they cheer the Orton's and Jericho's of the world but not him? Why should he put himself on the line for the fans if they don't even respect him? And even though that's a textbook heel turn, is there any other superstar that it would ring true for more than John Cena? Even though most people don't hate the actual John Cena, he would essentially be pointing out what is legitimately true in terms of his John Cena character.
I think he would do an absolutely brilliant job and would build his heel character up quite a bit. From there, you can even set up a rematch between a heel John Cena and a face Undertaker at Summerslam. All in all, I know it is a big leap to make such a significant change, but in the grand scheme of things, it could also lead to something quite special.